1. Field of the Invention
The present invention relates to a process for preparing crystalline molecular sieves, particularly large pore zeolites, using a 3,7-diazabicyclo[3.3.1]nonane templating agent. More particularly, the process comprises contacting active sources of one or more oxides selected from the group consisting of monovalent element oxides, divalent element oxides, trivalent element oxides, tetravalent element oxides, and pentavalent element oxides with an organocation templating agent derived from a 3,7-diazabicyclo[3.3.1]nonane
2. State of the Art
Natural and synthetic crystalline molecular sieves are useful as catalysts and adsorbents. Each crystalline molecular sieve is distinguished by a crystal structure with an ordered pore structure, and is characterized by a unique X-ray diffraction pattern. Thus, the crystal structure defines cavities and pores which are characteristic of the different species. The adsorptive and catalytic properties of each crystalline molecular sieve are determined in part by the dimensions of its pores and cavities. Accordingly, the utility of a particular molecular sieve in a particular application depends at least partly on its crystal structure.
In general, molecular sieves are prepared by crystallization in an aqueous reaction mixture containing an organic templating agent, such as a nitrogen-containing organocation. By varying the synthesis conditions and composition of the reaction mixture different zeolites can be formed.
The choice of the organocation templating agent is believed to play an important role in the process of molecular sieve crystallization. Organic amines and quaternary ammonium cations were first used in the synthesis of zeolites in the early 1960s as reported by R. M. Barrer and P. J. Denny in J. Chem. Soc. 1961 at pages 971-982. This approach led to a significant increase in the number of new zeolitic structures discovered as well as an expansion in the boundaries of composition of the resultant crystalline products.
Previously, products with low silica to alumina ratios (SiO.sub.2 /Al.sub.2 O.sub.3 .ltoreq.10) had been obtained, but upon using the organocations as components in the starting gels, zeolites with increasingly high SiO.sub.2 /Al.sub.2 O.sub.3 were realized. Some of these materials are summarized by R. M. Barrer 1982, Hydrothermal Chemistry of Zeolites, New York: Academic Press, Inc.
It has been postulated that the positive charge of the organocation templating species (and its sphere of hydration) interacts favorably with negatively charged silicate subunits, resulting in the crystallization of the resultant molecular sieve. An example of such templating involves the crystallization of sodalite in the presence of tetramethylammonium (TMA) cation as reported by Ch. Baerlocher and W. M. Meier, 1969, Helv. Chimica Acta 52, 1853. The TMA cations are found within the cavities of the sodalite cages, yet the 6.9 .ANG. diameter of the cation (7.3 .ANG. when hydrated) precludes it from entering the cavity via the 6-membered ring portals after formation of the structure; therefore the sodalite cage must result from growth around the cation.
Unfortunately, the relationship between structure of the organocation and the resultant zeolite is far from predictable, as evidenced by the multitude of products which can be obtained using a single quaternary ammonium salt as reported by S. I. Zones et al., 1989, ZeoIites: Facts, Figures, Future, ed. P. A. Jacobs and R. A. van Santen, pp. 299-309, Amsterdam: Elsevier Science Publishers., or the multitude of organocations which can produce a single zeolitic product as reported by R. M. Barrer, 1989, Zeolite Synthesis, ACS Symposium 398, ed. M. L. Occelli and H. E. Robson, pp. 11-27, American Chemical Society.
Thus, it is known that organocations exert influence on the zeolite crystallization process in many unpredictable ways. Aside from acting in a templating role, the organic cation's presence also greatly affects the characteristics of the gel. These effects can range from modifying the gel pH to altering the interactions of the various components via changes in hydration (and thus solubilities of reagents) and other physical properties of the gel. Accordingly, investigators have now begun to consider how the presence of a particular quaternary ammonium salt influences many of these gel characteristics in order to determine more rigorously how such salts exert their templating effects.
It has been noted that many of the organocations which have been used as templates for zeolite synthesis are conformationally flexible. These molecules can adopt many conformations in aqueous solution, therefore several templates can give rise to a particular crystalline product. Studies which involved alterations on such conformationally flexible organic amines and cations have been published. For example, one study, Rollmann and Valyocsik, 1985, Zeolites 5, 123, describes how varying the chain length for a series of .alpha..omega.-linear diamines resulted in different intermediate-pore products. It has also been recently reported by M. D. Shannon et al., 1991. Nature 353, 417-420 that three different products which have related framework topologies, can be formed from three linear bis-quaternary ammonium templates of varying chain lengths.
Altering the structure of a conformationally rigid organic molecule can also lead to a change in the zeolite obtained, presumably due to the differing steric demands of each template. S. I. Zones, 1989, Zeolites 9, 458-467 reported that in switching from 1,3-dimethylimidazolium hydroxide to 1,3-diisopropylimidazolium hydroxide as template, using the same starting gel (SiO.sub.2 /Al.sub.2 O.sub.3 =100), the former directs toward formation of ZSM-22 whereas the latter affords ZSM-23. Further investigations of the influence of conformationally constrained templating agents are reported in Y. Nakagawa, et al. Synthesis of Microporous Materials, Volume I, Chapter 16, M. Occelli, H. Robson, ed.; Van Nostrand Reinhold, New York, 1992.
In summary, a variety of templates have been used to synthesize a variety of molecular sieves, including zeolites of the silicate, aluminosilicate, and borosilicate family. However, the specific utility of a given template is at present unpredictable. In fact, the likelihood of any given organocation to serve as an effective templating agent useful in the preparation of molecular sieves is conjectural at best.
Despite the unpredictable nature of molecular sieve templates, educated trial and error has led to the discovery of a few notable cyclic-organocation templating agents. For instance, the use of N,N,N-trimethyl cyclopentylammonium iodide in the preparation of Zeolite SSZ-15 molecular sieve is disclosed in U.S. Pat. No. 4,610,854 issued on Sep. 9, 1986, to Zones; use of 1-azoniaspiro [4.4]nonyl bromide and preparation of a molecular sieve termed "Losod" is disclosed in Hel. Chim Acta (1974), Vol. 57, page 1533 (W. Sieber and W. M. Meier); use of 1, .omega.-di(1-azoniabicyclo [2.2.2.] octane) lower alkyl compounds in the preparation of Zeolite SSZ-16 molecular sieve is disclosed in U.S. Pat. No. 4,508,837 issued on Apr. 2, 1985 to Zones; use of N,N, N-trialkylammonio-1-adamantanes in the preparation of Zeolite SSZ-13 molecular sieve is disclosed in U.S. Pat. No. 4,544,538 issued on Oct. 1, 1985 to Zones. U.S. Pat. No. 5,053,373 issued on Oct. 1, 1991 to Zones discloses preparing SSZ-32 with an N-lower alkyl-N'-isopropyl-imidazolium cation templating agent. U.S. Pat. No. 5,106,801 issued on Apr. 21, 1992 to Zones, et al. discloses a cyclic quaternary ammonium ion, and specifically a tricyclodecane quaternary ammonium ion, for the preparation of the metallosilicate Zeolite SSZ-31. U.S. Pat. No. 4,910,006 issued on Mar. 20, 1990 to Zones, et al. teaches using a hexamethyl[4.3.3.0]propellane-8,11-diammonium cation for the preparation of SSZ-26.
Specific aza-polycyclics have also been disclosed for use as templating agents for various crystalline materials. For example, EP 0193282 discloses a tropinium cation for preparing clathrasil zeolite ZSM-58. Similarly, use of quinuclidinium compounds to prepare a zeolite termed "NU-3" is disclosed in European Patent Publication No. 40016. U.S. Pat. No. 4,285,922 issued on Aug. 25, 1981 to Audeh, et al. discloses preparing ZSM-5 using 1-alkyl-4-aza-1-azonia-bicyclo]2.2.2]octane-4-oxide halides; and U.S. Pat. No. 3,692,470 issued on Sep. 19, 1972 discloses preparing ZSM-10 using 1,4-dimethyl-1,4-diazoniabicyclo[2.2.2]octane. The use of sparteine salts as templating agents for SSZ-24 is disclosed in commonly assigned concurrently filed U.S. patent application, Ser. No. 07/959,001, entitled "A PROCESS FOR PREPARING MOLECULAR SIEVES USING A SPARTEINE TEMPLATE". The use of 1,3,3,8,8-pentamethyl-3-azoniabicyclo [3.2.1]octane salts as templating agents for novel large pore zeolites is disclosed in commonly assigned concurrently filed U.S. patent application, Ser. No. 07/958,832, entitled "A PROCESS FOR PREPARING MOLECULAR SIEVES USING A 1,3,3,8,8-PENTAMETHYL-3-AZONIABICYCLO[3.2.1]OCTANE TEMPLATE". The use of 9-azabicyclo[3.3.1]nonane salts as templating agents for SSZ-24 and other large pore zeolites is disclosed in commonly assigned concurrently filed U.S. patent application Ser. No. 07/958,634, entitled "A PROCESS FOR PREPARING MOLECULAR SIEVES USING 9 -AZABICYCLO[3.3.1]NONANE TEMPLATES".
Thus, organocation templating agents have been used to prepare many different combinations of oxides with molecular sieve properties, with silicates, aluminosilicates, aluminophosphates, borosilicates and silicoaluminophosphates being well known examples.